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Diode propulsion could power microbots

By Justin Mullins

A new form of propulsion that could allow microrobots to explore human bodies has been discovered. The technique would be used to power robots and other devices such as microfluidic pumps from a distance.

Finding a propulsion mechanism that works on the microscopic scale is one of the key challenges for developing microrobots. Another is to find a way to supply such a device with energy because there is so little room to carry on-board fuel or batteries.

Now a team lead by Orlin Velev at North Carolina State University in Raleigh, US, has found that a simple electronic diode could overcome both these problems. Velev and Vesselin Paunov from the University of Hull, UK, floated a diode in a tank of salt water and zapped the set-up with an alternating electric field.

The field induced a current within the diode, much in the same way that a radio signal induces a current in an RFID (radio-frequency identification) chip. This in turn set up an electric field between the diode’s electrical contacts and created the propulsive force.

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The force occurs because the field accelerates ions in the water in one direction, pushing the diode in the other, a phenomenon known as electro-osmosis. “We reached speeds of several millimetres per second,” says Velev. He has even attached a number of diodes together to make a ring that rotates. “We could use these devices as pumps inside microfluidic devices,” he says.

Fundamental barrier

Others are impressed by the work. “It’s a very neat idea,” says Gareth McKinley, a mechanical engineer at the Massachusetts Institute of Technology in Cambridge, US. “It’s the kind of work where you think ‘I wish I’d thought of that’.”

But there are still significant challenges ahead. Velev’s diodes are millimetre-sized but any robot designed to work within the human body would have to be an order of magnitude smaller. In the past, attempts to shrink propulsive mechanisms have run up against a fundamental barrier in fluid dynamics&colon; fluids become progressively more viscous on smaller scales. “It’s like moving through honey,” says Velev.

But extrapolations of the team’s measurements indicate the propulsive force will work just as well at smaller scales. “The propulsive force scales in exactly the same way as the drag. That’s quite significant,” says McKinley.

Another challenge is that electro-osmosis occurs only at higher pH levels, when the ionic content of the water is high. Changing the pH from acidic to alkaline reverses the direction of thrust and there is zero thrust when the pH is about 6. Blood is only weakly alkaline so Velev will have to make adjustments to generate significant propulsive forces inside the body. He thinks the problem might be overcome by covering the diode with a polymer that shifts the pH at which zero thrust occurs.

McKinley believes the technique has potential because the electric field can also provide power for onboard sensors that monitor things like temperature and pH. It might even be possible to power logic operations that make decisions on the basis of these measurements. Velev has already demonstrated how the field can power light-emitting diodes, so that different types of propelled robots can be easily distinguished by their colour.

LEDs also offer the team a way to control a diode independently if several are operating in the same electric field. Switching off the field causes them all to stop. But shining a laser spotlight onto an individual LED causes it to grind to a halt while the others keep moving, because the photons interfere with electron transport within the LED, essentially switching it off.

Pressure gradients

Other types of micropropulsion have all run up against significant barriers. One idea exploits the phenomenon in which an electric current in a magnetic field experiences a force. The idea is to bathe a robot in a magnetic field and then switch on a current to generate a force. “The trouble is you need to power the current, which requires an onboard battery. How do you do that?” asks McKinley.

Ultrasound can create pressure gradients within liquids that can move particles around. The problem here is that ultrasound can be hard to focus and can also cause bubbles to form and collapse, a process called cavitation that can damage cells.

Yet another option is to carry an onboard supply of hydrogen peroxide, which dissociates into steam and oxygen. Expelling these gases generates a force – the attitude thrusters on the space shuttle work in the same way. But the fuel uses up space that would otherwise be available for sensors.

“Diode propulsion has potential advantages over all these,” says McKinley.